The adsorption of copper and cobalt from aqueous solution on to illite and other substrates has been studied as a function of pH, solution composition and solid phase concentration. The results are interpreted in terms of a model whereby the trace metals are adsorbed in exchange for surface bound H + ions. Adsorption varies with solution ionic strength and the concentrations of complex forming ligands; both of these parameters tend to reduce the trace metal adsorption. The Cu 2+ is two orders of magnitude more reactive toward solid surfaces than Co 2+ , which is consistent with the general reactivities of these two metal ions. It is also found that Mg 2+ interferes with adsorption, presumably by competing with the trace metals for the surface sites. A quantitative model was developed which describes adsorption of these metals from natural waters ranging from river water to sea water as a function of pH, complexing ligands and magnesium activity.